@digta/mcprotocol

Mitsubishi MELSEC Communication (MC) protocol client for Ethernet PLCs, written entirely in JavaScript.

This is an @digta-scoped fork of mcprotocol 0.1.2 (by Dana Moffit) with a single behavioural fix: TCP frame reassembly for ASCII 3E responses. It is a drop-in replacement — no API changes. See CHANGELOG.md for the details.

Part of Protokit.

This software is not affiliated with Mitsubishi. FX3U and MELSEC are trademarks of Mitsubishi.

⚠️ BETA CODE. Wrong values can be written to wrong locations. Test everything thoroughly before using against a live PLC.

Install

npm i @digta/mcprotocol

What's different from upstream

[email protected] treated every socket data event as exactly one complete PLC response. TCP is a byte stream, so an ASCII 3E response can be split across several events — or several responses can arrive in one — which produced Invalid Response Length / DATA LESS THAN 11 BYTES errors and connection resets when polling many registers.

This fork buffers incoming bytes per connection and slices out whole frames using the response data-length field (18 + dataLen chars) before processing. It is active only for ascii: true + frame: '3E'; every other mode uses the original code path. Full write-up in CHANGELOG.md.

Usage

var mc = require('@digta/mcprotocol');
var conn = new mc;

var variables = {
  TEST1: 'D0,5',        // 5 words starting at D0
  TEST4: 'R2000,2',     // 2 words at R2000
  TEST6: 'D6000.1,20',  // 20 bits starting at D6000.1
};

conn.initiateConnection({ port: 1281, host: '192.168.0.2', ascii: false }, connected);

function connected(err) {
  if (typeof err !== 'undefined') { console.log(err); process.exit(); }
  conn.setTranslationCB(function (tag) { return variables[tag]; });
  conn.addItems(['TEST1', 'TEST4']);
  conn.addItems('TEST6');
  conn.readAllItems(valuesReady);
}

function valuesReady(anythingBad, values) {
  if (anythingBad) { console.log('SOMETHING WENT WRONG READING VALUES'); }
  console.log(values);
  process.exit();
}

Note: to use the ASCII 3E fix, configure the connection with { ascii: true, frame: '3E' }.

API

• initiateConnection(params, callback) — connect. params: port, host, ascii (default false), frame ('1E' default, '3E' supported), octalInputOutput (default true). callback(err).
• dropConnection() — terminate the TCP connection.
• setTranslationCB(translator) — map tag names to addresses. translator(tag) returns an address string, e.g. M100,20, D2000,5, DFLOAT1000, RSTR30,10, D1000.2,5.
• addItems(items) / removeItems(items) — manage the read polling list (string or array of strings).
• writeItems(items, values, callback) — write values to the PLC.
• readAllItems(callback) — read the polling list; callback(anythingBad, values).

For the full address syntax and configurator setup notes, see the upstream mcprotocol documentation.

Tested against

Direct connection to FX3U-ENET / FX3U-ENET-ADP. Q-series E71 should work in theory (same frames). Serial and UDP are not supported.

Changelog

1.0.0 — fork of [email protected] with the ASCII 3E TCP frame reassembly fix. Full details in CHANGELOG.md.

License

MIT — © 2015 Dana Moffit (original), © 2026 Rifky Andigta Al-Fathir (fork modifications).